Kidney Week

Abstract: TH-PO1022

Exploring the Exchangeability of Transmembrane Segments between Na/HCO3 Cotransporter NBCe1-A and Cl-HCO3 Exchanger AE1

Session Information

• Acid Base: Basic
  November 02, 2017 | Location: Hall H, Morial Convention Center
  Abstract Time: 10:00 AM - 10:00 AM

Category: Fluid, Electrolytes, and Acid-Base

• 701 Acid-Base: Basic

Authors

• Lee, Seong-Ki, Case Western Reserve University, Cleveland, Ohio, United States

Seong-Ki Lee,

• Boron, Walter F., Case Western Reserve University, Cleveland, Ohio, United States

Walter F. Boron,

Background

The SLC superfamily of transport proteins comprises >300 genes in 52 families. Here we study 2 SLC4 family members: electrogenic NBCe1 (SLC4A4) and electroneutral Cl-HCO₃ exchanger AE1 (SLC4A1). NBCe1-A is predominantly expressed in the renal proximal tubule basolateral membrane, where it reabsorbs HCO₃^– into the blood. The erythrocyte variant of AE1 is important for CO₂ carriage in blood. Recently, Arakawa et al. published the crystal structure of human AE1, which has 14 transmembrane segments (TMs). They classified the TMs into a core domain (CD; 8 TMs) and a gate domain (GD; 6 TMs). They proposed that conformational changes of the GD allow transport of substrate, whereas the CD provides substrate specificity.

Methods

Guided by this new AE1 structure, we defined NBCe1’s TMs (~25-30% amino-acid identity to AE1). We hypothesized that NBCe1-A will work when implanting gate TMs of AE1 into NBCe1-A, whereas NBCe1-A will not work when implanting core TMs of AE1 into NBCe1-A. We generated 14 NBCe1-A chimeras, 1 for each TM. We assessed NBCe1-A’s electrogenic activity by two-electrode voltage-clamping Xenopus oocytes expressing each mutant. We checked surface expression (SExp) by biotinylation.

Results

Data is summarized in Table. We found that, in general, implanting gate TMs induces SExp, whereas implanting core TMs does not. We also found that some mutants show robust electrogenic activity (I_NBC, NBCe1-A current, or ΔV_m upon HCO₃⁻ addition).

Conclusion

Our findings suggest that the CD of NBCe1-A cannot tolerate swapping of individual TMs because of coordination with substrates, whereas the GD of NBCe1-A generally expresses after such swaps, and even shows robust electrogenic activity in some cases presumably because the swapped TM still works as a structural component.